The present invention relates generally to sensors for measuring forces employing surface acoustic waves, and more particularly to highly stable and sensitive hydrostatic pressure sensors, suitable for high pressure applications, employing surface acoustic waves.
Sensors employing surface acoustic wave (hereinafter "SAW") devices such as delay lines and resonators are known for measuring accelerations, stresses and strains, and pressure. These sensors generally are based on the propagation of surface acoustic waves across a thin, flexible diaphragm which is deformed when subjected to an applied acceleration, stress or strain, or pressure. The surface acoustic wave delay time is a function of the applied external acceleration, stress or strain, or pressure, since the wave velocity and path length vary with diaphragm deformation. The change in surface acoustic wave propagation characteristics is measured as a change in the frequency of oscillation of external oscillator circuitry connected in series with the SAW device in a regenerative feedback loop. U.S. Pat. No. 3,978,731, issued Sept. 7, 1976 to Reeder et al and U.S. Pat. No. 3,863,497, issued Feb. 4, 1975 to van de Vaart et al. disclose such SAW sensors.
Several approaches to making the pressure-sensitive diaphragm of a SAW sensor are known. A sensor having piezoelectric transducers deposited by thin film techniques on a steel beam is disclosed in U.S. Pat. No. 4,107,626, issued Aug. 15, 1978 to Kiewit. A sensor having dual substrates, a SAW substrate and a base substrate, of the same material and orientation bonded to one another is disclosed in U.S. Pat. No. 4,216,401, issued Aug. 5, 1980 to Wagner. Such sensors have severely restricted operating characteristics or are subject to deteriorating performance or actual failure due to limitations of the bond.
A pressure-sensitive diaphragm may also be formed by boring or drilling a central cavity in the SAW substrate, as disclosed for example in U.S. Pat. No. 4,100,811, issued July 18, 1978 to Cullen et al. While this approach avoids the use of a bond in the sensitive region, bored or drilled diaphragms of this type are not readily fabricated to a desired thickness or to a very thin thickness, or with parallel membrane surfaces. Additionally, sharp and deep corners are encountered which lead to stress concentrations which limit such sensors to low pressure applications.
A cylindrical pressure sensing diaphragm which avoids some of the difficulties mentioned above is disclosed in a U.S. Pat. No. 3,878,477, issued Apr. 15, 1975 to Dias et al. Respective end caps are provided to admit a fluid into the interior of the diaphragm to effect the pressure measurement. Such a cylindrical diaphragm is disadvantageous, however, in that variations in temperature adversely affect the pressure measurement.
In general, sensors utilizing SAW devices, including the cylindrical pressure sensing diaphragm of the Dias et al patent, are adversely affected by temperature variations. Such SAW devices generally comprise a SAW substrate of such piezoelectric materials as quartz, lithium niobate, and lithium tantalate, or a composite treated substrate such as silicon having a suitable thin film coating of piezoelectric material such as zinc oxide, all of which exhibit sufficient acousto-electric coupling to provide a measurable variation in surface acoustic wave propagation velocity in response to variations in the subsurface strain thereof. Since these materials are sensitive to strain-related phenomena which include temperature as well as stress and acceleration, pressure sensors either must include means for compensating for temperature variations or be operated at a given temperature or over a narrow given temperature range if a temperature compensated orientation such as the ST cut ((yxwl) 0.degree./42.75.degree.) or the SST cut ((yxwl) 0.degree./-49.22.degree., propagation direction of 23.degree. from the digonal axis) is used.
Some techniques for compensating for temperature variations in various types of sensors are known. The aformentioned Kiewit patent discloses a temperature compensation technique in which surface acoustic waves travel in adjacent regions of essentially the same generally planar surface so that the effect of temperature variations on the respective regions is substantially equal. With force applied to the sensor, a difference frequency obtained by mixing the outputs of the respective oscillators associated with the regions, one of which is in compression and the other of which is in tension, is proportional to the deflection of the beam within its elastic limits. The aforementioned Dias et al patent discloses a temperature compensation technique in which dual acoustic surface wave oscillators coupled to a single generally planar substrate of piezoelectric material inversely change their respective frequencies in response to a force applied normal to the surface of the substrate. The aforementioned Reeder et al patent discloses a temperature compensation technique in which the two acoustic channels of the sensor are fabricated close together on the same substrate of a generally planar diaphragm so that their temperature difference will tend to be small. One of the channels is a primary, or measurement channel, and the other is a reference channel. The reference channel pressure is held constant so that the output of the measurement channel, after being mixed with the output of the reference channel, is a guage of the absolute pressure. U.S. Pat. No. 3,886,484, issued May 27, 1975 to Dias et al. discloses devices in which two delay lines, one having a rotated Y cut of .theta.=42.75.degree. and the other having a rotated Y cut of R=35.degree., are cascaded to provide a broader temperature range of stable operation. U.S. Pat. No. 3,999,147, issued Dec. 21, 1976 to Otto et al., discloses an acoustic wave device having reflective gratings combined with a material such that the temperature coefficients of delay along different directions are of opposite sign. The acoustic wave is propagated along suitable path lengths to provide a linear zero temperature coefficient of delay.